Re: [PATCH v6 5/6] MCS Lock: Restructure the MCS lock defines andlocking code into its own file

[Tim Chen]

On Fri, 2013-09-27 at 08:29 -0700, Paul E. McKenney wrote:
> On Wed, Sep 25, 2013 at 03:10:49PM -0700, Tim Chen wrote:
> > We will need the MCS lock code for doing optimistic spinning for rwsem.
> > Extracting the MCS code from mutex.c and put into its own file allow us
> > to reuse this code easily for rwsem.
> > 
> > Signed-off-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
> > Signed-off-by: Davidlohr Bueso <davidlohr@xxxxxx>
> > ---
> >  include/linux/mcslock.h |   58 +++++++++++++++++++++++++++++++++++++++++++++++
> >  kernel/mutex.c          |   58 +++++-----------------------------------------
> >  2 files changed, 65 insertions(+), 51 deletions(-)
> >  create mode 100644 include/linux/mcslock.h
> > 
> > diff --git a/include/linux/mcslock.h b/include/linux/mcslock.h
> > new file mode 100644
> > index 0000000..20fd3f0
> > --- /dev/null
> > +++ b/include/linux/mcslock.h
> > @@ -0,0 +1,58 @@
> > +/*
> > + * MCS lock defines
> > + *
> > + * This file contains the main data structure and API definitions of MCS lock.
> > + */
> > +#ifndef __LINUX_MCSLOCK_H
> > +#define __LINUX_MCSLOCK_H
> > +
> > +struct mcs_spin_node {
> > +	struct mcs_spin_node *next;
> > +	int		  locked;	/* 1 if lock acquired */
> > +};
> > +
> > +/*
> > + * We don't inline mcs_spin_lock() so that perf can correctly account for the
> > + * time spent in this lock function.
> > + */
> > +static noinline
> > +void mcs_spin_lock(struct mcs_spin_node **lock, struct mcs_spin_node *node)
> > +{
> > +	struct mcs_spin_node *prev;
> > +
> > +	/* Init node */
> > +	node->locked = 0;
> > +	node->next   = NULL;
> > +
> > +	prev = xchg(lock, node);
> > +	if (likely(prev == NULL)) {
> > +		/* Lock acquired */
> > +		node->locked = 1;
> > +		return;
> > +	}
> > +	ACCESS_ONCE(prev->next) = node;
> > +	smp_wmb();

BTW, is the above memory barrier necessary?  It seems like the xchg
instruction already provided a memory barrier.

Now if we made the changes that Jason suggested:


        /* Init node */
-       node->locked = 0;
        node->next   = NULL;
 
        prev = xchg(lock, node);
        if (likely(prev == NULL)) {
                /* Lock acquired */
-               node->locked = 1;
                return;
        }
+       node->locked = 0;
        ACCESS_ONCE(prev->next) = node;
        smp_wmb();

We are probably still okay as other cpus do not read the value of
node->locked, which is a local variable.

Tim

> > +	/* Wait until the lock holder passes the lock down */
> > +	while (!ACCESS_ONCE(node->locked))
> > +		arch_mutex_cpu_relax();
> > +}
> > +
> > +static void mcs_spin_unlock(struct mcs_spin_node **lock, struct mcs_spin_node *node)
> > +{
> > +	struct mcs_spin_node *next = ACCESS_ONCE(node->next);
> > +
> > +	if (likely(!next)) {
> > +		/*
> > +		 * Release the lock by setting it to NULL
> > +		 */
> > +		if (cmpxchg(lock, node, NULL) == node)
> > +			return;
> > +		/* Wait until the next pointer is set */
> > +		while (!(next = ACCESS_ONCE(node->next)))
> > +			arch_mutex_cpu_relax();
> > +	}
> > +	ACCESS_ONCE(next->locked) = 1;
> > +	smp_wmb();
> 
> Shouldn't the memory barrier precede the "ACCESS_ONCE(next->locked) = 1;"?
> Maybe in an "else" clause of the prior "if" statement, given that the
> cmpxchg() does it otherwise.
> 
> Otherwise, in the case where the "if" conditionn is false, the critical
> section could bleed out past the unlock.
> 
> 							Thanx, Paul
> 


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